Method for increasing the dynamic stability under load of a toothed structural component

ABSTRACT

A method is proposed for increasing the carbon-content dependent dynamic stability of a toothed structural part, particularly of a toothed wheel. A curve of the carbon content portion in an edge layer of the structural part is adjusted so that the carbon portion, departing form the surface in direction to a core of the structural part, at first increases and as of a certain depth of the layer continuously decreases. Part of the edge of the layer is removed in areas of the structural part whose dynamic stress capacity increases as the carbon content increases on the surface of the structural part.

[0001] The invention relates to a method for increasing the dynamic stability dependent on the carbon content of a toothed structural component made of case hardenable steel.

[0002] It is known from the practice to increase the dynamic stability of structural parts by producing a wear-resistant edge layer during a case hardening operation a sufficiently tough core. For that purpose, the edge layer is carburized and the structural component subsequently hardened. For mechanically highly stressed structural components, especially toothed wheels and shaft, the case hardening of alloyed case hardened steels represents a solution with which the greatest stability is achieved.

[0003] In ISO 6336-5, First Edition, Jun. 15, 1996, a carbon content is established on the surface which is +0.2 to −0.1% from the eutectoid point of the phase diagram of case hardened steels and ensures great dynamic stability. Thereby results, especially for chromium-nickel case hardened steels, a preferred carbon content of substantially 0.8%.

[0004] It is further known from the practice that the carbon content on the surface and in the edge layer affects the stability of tooth root and tooth flank of a toothed wheel in different manners. For chromium-nickel case hardened steels, it is known that a maximum tooth root load capacity exists with a carbon content of 0.6% on the surface of a structural part, which decreases as the carbon content increases. On the other hand, a tooth flank load capacity steadily increases as the edge carbon content increases from 0.6% to 0.9%.

[0005] With the known serial thermal treatment processes of chromium-nickel case hardened steels, there results the disadvantage that an adjustment of a maximum or minimum tooth flank load capacity leads to a reduction of the tooth root load capacity, since during a case hardening process, a carbon content is substantially uniformly adjusted in the edge layer of the whole structural part. When the carbon content in the edge layer of a toothed wheel is adjusted so that a maximum of the tooth root load capacity exists, this goes, in turn, to oppressing the tooth flank load capacity.

[0006] To overcome the disadvantage in highly loaded toothings, a change has been made to maximizing the tooth flank load capacity via the carbon content in the edge zone and increasing the tooth root capacity of the toothings by a subsequent strengthening radiation in the area of the tooth root.

[0007] It is a disadvantage, however, that considerable additional expenses result from the strengthening radiation of the root of toothings following the case hardening.

[0008] The problem on which this invention is based is, therefore, to make a method available for increasing the carbon-content dependent stability of a toothed structural part made of case hardenable steel, which makes subsequent strengthening radiation unnecessary and makes an economical production of a structural part possible or the increase of the dynamic stability.

[0009] According to the invention this problem is solved with a method comprising the features of claim 1.

[0010] The inventive method for increasing the dynamic stability dependent on carbon content of a toothed structural part, such as a toothed wheel made of case hardenable steel, wherein the curve of the carbon content in the edge layer of the structural part is first adjusted so that the carbon content, starting from the surface, first increases in direction of the core of the structural part and as of a certain depth of the layer continuously decreases and in which a part of the edge layer is then removed in areas of the structural part, specially the area of the tooth flanks, whose dynamic stability increases as the carbon content rises on the surface of the structural part, offers the advantage that on the surface of the structural part an optimized edge carbon content exists which always ensures a maximum value of the load capacity of the structural part concerned.

[0011] With the inventive adjustment of the carbon content and subsequent hardening process of specific areas of the structural part, it is advantageously possible to omit a refining treatment that increases the load capacity and results in an increase of the production costs.

[0012] A weight reduction is also possible, because of the increased strength of the structural part treated with the inventive method, since less case hardening material is needed for the same strength.

[0013] Other advantages and advantageous developments of the object of the invention result from the following description, claims and drawing.

[0014] In the single FIGURE of the drawing are compared a curve 2 of the carbon content in the edge layer of a structural part produced according to the inventive method and another curve 1 having a carbon content of another part produced by the case hardening method known from the practice.

[0015] The two curves 1, 2 of the carbon content in the edge layer of a toothed wheel are shown from the surface thereof toward a core. The carbon content (in percent) is here plotted via the edge distance from the surface of the toothed wheel ( in millimeters).

[0016] The curve 1 reproduces a carbon content C in the edge layer of the toothed wheel which was produced by a conventional case hardening method. Said curve 1 has a maximum of the carbon content C in the intermediate area of the surface of the toothed wheel and the carbon content C decreases steadily and at least approximately linearly as the edge distance increases.

[0017] In the edge zone of a toothed wheel, the curve 2 of the carbon content C is adjusted with decarburization of the edge by means of the modified case hardening method, and is designed so that the carbon content, departing from the surface of the toothed wheel, at first increases in direction of the core and as of a certain depth of the layer, in this case, for example, at an edge distance of about 0.15 mm, has a maximum of carbon of 0.7%. Starting from said maximum, the carbon content continuously decreases in direction of the core of the toothed wheel. The so-called edge decarburization is adjusted or produced by a sharp decrease of the carbon content of the atmosphere of the carburization agent in the last step of the case hardening method, for example, while the hardening temperature is maintained.

[0018] The inventively modified case hardening method is described, in detail, herebelow by way of example.

[0019] During the whole case hardening process, the toothed wheel made of case hardenable steel, for example, 18 CrNiMo 88, is surrounded in a heating furnace by a gaseous carburization agent whose carbon content is changed during the individual phases of the case hardening method.

[0020] The case hardening method comprises the phases known per se, namely, a heat-up phase, a carburization phase, a cooling phase and a holding phase at hardening temperature of the structural part, said consecutive phases each being carried out over a specific time at preset temperatures and different carbon content of the gas atmosphere.

[0021] It is thus common practice that the carbon content at first, during the heat-up phase, is less than it is during the carburization phase that follows. In the heat-up phase the carbon content can amount for 0.4%, for example.

[0022] In a manner known already and preferred, the carburization phase that follows is divided into four sections or steps, the carbon content in the first three steps being raised from 0.4% gradually to 0.6% and finally to a value of 1.1%, it being possible that the time interval in which a carbon content of 1.1% is an adjusted amount to fifteen times the time interval in which the carbon content of 0.6% is adjusted. In a last section or step, brief from the time point of view of the carburization phase, said content is again reduced to a value of 1.0%. In the embodiment described, the temperature of the heat-up phase and of the carburization phase is about 930° C.

[0023] During the cooling phase following the carburization phase, the carbon content corresponds to the last section or the last period of the carburization phase.

[0024] The holding phase during which the toothed wheel is held at hardening temperature, in turn, divides itself into several sections or steps, the carbon content of the atmosphere of the gaseous carburization agent in a first section is adjusted to a value of 0.4%. In a second section of the holding phase, the carbon content of the carburization agent is lowered to 0.3%, the second section of the holding phase comprises a longer period of time than the first section of the holding phase and can last, for example, almost twice as long as the first section. The temperature of the cooling phase and of the holding phase substantially corresponds to 860° C.

[0025] It is to be noted here that the individual, numerically expressed percent parameters of the case hardening method and the duration of the individual phases depend on the furnace type respectively used and vary accordingly, the tendencies in the different types of furnace used remaining the same.

[0026] After the holding phase, the toothed wheel is hardened directly in oil and then left at 170° C. for 90 minutes.

[0027] After the case hardening, the toothed structural part, such as a toothed wheel, is processed hard in the area of the tooth flanks, the material removal of the edge zone substantially corresponding to the area of the edge cooling. As processing method is preferably provided here a grinding process which, in the practice, has hitherto been provided for improving the excellence of the surface of the tooth flanks. The area of the tooth root is, as a rule, not subsequently processed. This means that the costly strengthening radiation, known from the prior art, can be avoided and thus no additional expenses result compared with the conventional production method.

[0028] The curve 2 of the carbon content in the whole edge area of the toothed wheel is adjusted with the case hardening method described above. This means that both in the area of the tooth flank and in the area of the tooth root, the carbon content on the surface is substantially from 0.5% to 0.6%. When the carbon content is 0.6% on the surface of the toothed wheel, the tooth root load capacity reaches its maximum value. A higher carbon content in steels, especially CrNi steels, results in a reduction of the tooth root load capacity. Thus a maximum value for the load capacity of the tooth root is achieved with the case hardening method with edge decarburization.

[0029] The load capacity of the tooth flank has its maximum value with a carbon content in the range of from 0.6% to 0.9%, the tooth flank load capacity steadily increasing from a carbon content of 0.6% as the carbon portion in the edge zone increases. This means that prior to the removal of material, a tooth flank load capacity has a value below a maximum when the carbon content in the area of the surface of the tooth flank is of 0.6%. By the removal of material, the edge layer is diminished so that the layer areas of the edge layer of the tooth flank form the surface of the toothed wheel and have a higher carbon content than the areas of the tooth root near the surface. The tooth flank load capacity of the conventional case hardening is thus again achieved. A maximum tooth root load capacity is not affected by the wear of the tooth flank.

[0030] Thus, after the case hardening modified, according to the invention, which constitutes a direct hardening with edge decarburization, and the hard processing that follows of the tooth flank, optimized edge carbon contents exist which produce a maximum load capacity of the tooth flank and of the tooth root.

[0031] According to the above described modified case hardening method with edge decarburization in the last step of the case hardening, there are clearly reached higher values of the dynamic breaking forces than according to conventional case hardening methods. By edge decarburization, together with an increase of the fatigue limit, the curve of the endurance strength of toothed wheels is also moved to higher vibration cycle criteria.

[0032] With the modified direct case hardening method with edge decarburization is also improved the resistance to vibration under increasing bending stress and the dynamic breaking strength in case of sudden stress in contrast with the toothed wheels hardened by known serial thermal treatment methods.

[0033] Although the inventive method is suited particularly to increasing the tooth root load capacity in toothed structural parts, especially toothed wheels, the invention is not limited to said application. The above described hardening method with subsequent hard processing is also adequate for shafts for steering systems or industrial vehicles and for any other structural parts which are exposed to high dynamic stress and must have a hard, wear resistant edge layer on a sufficiently tough core.

[0034] Reference Numerals

[0035]1 curve of the carbon content in an edge layer during a conventional case hardening method

[0036]2 curve of the carbon content in an edge layer during a modified, inventive case hardening method with edge decarburization. 

1-5. (canceled)
 6. A method for increasing a dynamic stability dependent on a carbon content of a toothed structural part made of case hardenable steels, wherein a curve (2) of the carbon content in an edge layer of the toothed structural part is first adjusted so that the carbon content, departing from a surface in direction of a core of the structural part, at first increases and as of a certain depth of the edge layer continuously decreases, and that subsequently one part of the edge layer in areas of the toothed structural part, particularly in an area of a tooth flank, is removed so that a maximum value of the carbon content be on the surface of the structural part.
 7. The method according to claim 6, further comprising the step of producing said curve (2) of the carbon content in the edge layer of the structural part by means of carburization.
 8. The method according to claim 7, surrounding, during carburization, the structural part by a gaseous carburizing agent and varying the carbon content of the atmosphere.
 9. The method according to claim 8, further comprising the step of adjusting the carbon content of the atmosphere during a holding phase that follows a cooling phase, to a hardening temperature of the structural part substantially lower than during the carburization.
 10. The method according to claim 6, further comprising the step of removing a part of the edge layer to be removed on the tooth flank by means of mechanical processing.
 11. A method for increasing dynamic stability dependent on a carbon content of a toothed structural part made of case hardenable steels, the method comprising the steps of: adjusting a curve (2) of the carbon content in an edge layer of the structural part so that the carbon content at a surface increases and as of a certain depth of the layer continuously decreases departing from the surface in direction of a core of the structural part; and removing one part of the edge layer in areas of the structural part, particularly in the area of the tooth flank, so that a maximum value of the carbon content is at the surface of the structural part. 